Meteorological March Madness 2012

Disclaimer: This draft is an evolving research assessment and
not a final report.
The analyses presented have not yet been peer reviewed and do not
represent official positions of ESRL, NOAA, or DOC.
A more technical and detailed analysis of the heat wave has been
submitted
to the Bulletin of the American Meteorological Society for peer review.
Comments are welcome. For more information, contact
Dr. Martin Hoerling
(martin.hoerling@noaa.gov)

Physical Process Understanding

In order to determine the cause(s) of an extreme event, an understanding
of the physical processes is required. Here we present some preliminary
analyses to better understand what factors may have contributed to the
remarkable magnitude of this spring heatwave. These basic physical
considerations, while providing clues on causes, also allow one to
rank the likelihood of alternative explanations being true. A more
detailed assessment will ultimately be required, especially in regard
to a quantification of the plausible contributing factors.

It is useful to consider the heatwave as expressed in 850mb temperatures,
the pattern and magnitude of which was shown to be virtually identical to
that witnessed at the surface. We will return to the surface conditions
in subsequent analysis, where we consider how processes of snow cover
and soil moisture feedback could have affected the heat balance and
surface expressions of the heatwave magnitude.

Much of the heatwave magnitude in the lower troposphere can be reconciled
with the nearly horizontal transport of sensible heat, along the 850mb
surface, from the Gulf Coast region to the Canadian border. Fig. 5
(top) shows that the +15°C maximum departures at 850mb were located
over the northern Great Lakes region. The 850mb vector wind anomalies
were strongly southerly across a corridor of the eastern Great Plains
from Louisiana to the Canadian Prairie (Fig. 5, middle), with 12 m/s
anomaly magnitudes. The steady nature of this pattern during the
two week period of the heatwave allows one to infer the anomalous
air parcel trajectories from these time-averaged wind anomalies.
The anomalous streamlines, implied by the vector wind arrows in Fig. 5,
are thus a reasonable approximation to trajectories. Resulting air parcel
transit times from the Gulf to the Canadian border would be roughly on
the order of 1-2 days. Preliminary back trajectory calculations reveal
air parcel displacements strongly resembling the streamlines, as they
should when the flow pattern is quasi-stationary, but further analysis
will be required.

The intensity of the poleward heat transport can be inferred from the
superpositioning of these anomalous winds upon the background contours of
climatological temperatures. At 850mb, the climatological temperature
gradient (Fig, 5, bottom) reveals a 20°C difference between 850mb
temperatures on the Gulf Coast compared to the northern Great Lakes area.
Undiluted, quasi-adiabatic transport would thus be able to rapidly
exchange appreciable heat, and leading to much of the observed high
latitude warming on the order of what occurred. Of course, one would
expect some diabatic processes along the trajectory, but in the absence of
precipitation (and, indeed there was little rainfall during this period),
the air parcel motions would most probably approximate displacements along
isentropic rather than pressure surfaces. It is also reasonable that
some compressional heating from subsidence on the flank of the anomalous
500mb high would have affected parcel temperatures beyond their initial
heat content from the Gulf of Mexico source regions.

An appreciable fraction of the magnitude of the warming is thus
reconcilable with a simple transport of heat from Gulf Coast region
northward. One can infer from Fig. 5 that the maximum in horizontal
heat transport convergence (and thus, positive temperature tendency)
would be realized over the Midwest north of the largest 850mb wind
anomalies. While these are semi-quantitative statements, our initial
diagnosis that the essential physical process of the heatwave
development involved a strong anomalous poleward heat transport in the
region east of the Rocky Mountains during 12-23 March 2012 is irrefutable.

The 850mb and 500mb anomalies during 12-23 March 2012 are characteristic
atmospheric features of variability, ones that are effective in inducing
poleward heat transport and strong temperature variability over the
northern U.S. . They were, however, extreme circulation anomalies in
terms of magnitude and perhaps persistence.

Figure 6: Standard deviation of monthly March 850mb temperature (top), 500mb heights (middle) and 850mb wind velocity (bottom) of the period 1961 to 1990 (left) and the ratio of standard deviations between period 1991 to 2011 and period 1961 to 1990 (right). [Data source: NCEP/NCAR reanalysis]

Figure 7: Same as figure 6 but for daily March values. Contour intervals for the 1961 to 1990 climatological values (left panels) are doubled relative to climatologies based on monthly values in Figure 6.

Have these features of the time-averaged atmospheric circulation become
more "energetic" in the sense that the current 2012 unusual intensity
of atmospheric dynamics might be related to an increase in variability
over time? Perhaps the explanation is "weather weirding", "weather on
steroids", or an extreme non-linearity to assert how the atmosphere may
be responding to increases in greenhouse gases? Such explanations can
be tested with data and modeling experiments. Here we present some
preliminary analysis of March daily and monthly means. In summary,
our diagnoses provided below show no support for the supposition
that there has been enhanced variability, at least to date for the
particular variables of immediate relevance for the March 2012 heatwave.
Fig. 6 compares the variability of monthly averaged 850mb temperature
(top), 500mb height (middle), and 850mb wind (bottom). The variability
during 1961-1990 is presented in the left side panels, and the change
to 1991-2011 (expressed as a ratio) is shown in the right panels.
An identical analysis is provided in Fig. 7, but based on the variability
and its change from daily values. Neither the monthly or the daily data
reveal appreciable increase in the variability of March weather, though
this does not negate the possibility that such patterns may change in
the future. And, while finding no evidence for a change in variability,
it is nonetheless evident that the anomalies in 850mb temperatures, 850mb
winds, and 500 mb heights related to the heatwave were quite likely of
historic proportions for this time of year, with departures during 12-23
March 2012 on the order of 5 times the magnitude of typical variability.

Vertical profiles of air temperature, dew point temperature, and winds
using rawindsonde observations indicate the strong coupling between
conditions in the lower troposphere and conditions at the surface
(Fig. 8). Shown are a sequence of 00Z vertical profiles of temperature,
dewpoint, and winds taken at Minneapolis, MN from 7 March thru 21 March.
The strong veering of winds with height between the surface and 850mb
level is a well-known signature of strong warm air advection in the low
layer. Importantly, this layer was well-mixed as indicated by steep,
near dry-adiabatic lapse rates, and deep planetary boundary layers
as revealed by elevated lifting condensation levels. The implied
strong vertical mixing of air is precisely what would be expected
from the aforementioned similarity in magnitudes of the surface and
850mb temperature anomaly magnitudes. Of course, a major source of the
lower tropospheric high temperatures is transport indicated in Fig. 6.
One can also discern the presence of strong inversions in the soundings
at or below 700mb, indicative of subsident warming.

Figure 8: Vertical profiles of air temperature, dew point temperature, and winds using rawindsonde observations from Minneapolis, MN, March 7-21. [Data source: University of Wyoming]

The strong coupling of surface and the lower troposphere was thus a
key physical attribute of the heatwave. This fact does not negate a
role for land surface feedbacks. What might have been their role?
Regarding snow feedbacks, it is useful to first note that the March
climatology of snowcover, as provided by the Rutgers University Global
Snow Lab analysis (Fig. 9) reveals that the probability of having
snow cover in March--- as far north as Chicago-- is less than 20%.
In other words, most areas south of a latitude band near 42°N
are snow free in March, on average. Thus, the fact that the observed
temperature departures during the height of the heatwave in 2012 were
uniformly above +10°F along a line from Chicago southward to Memphis
cannot be explained by unique feedbacks associated with a lack of snow cover.
However, snowcover is common in March north to the Canadian border. While
snow did cover that region in advance of the heatwave (as shown in
the animation link below), its low water equivalence led to rapid
melt and sublimation.

As shown by our analysis of observational data, an explanation that this
heatwave was an outcome of a strong nonlinear feedback associated with
a climate change induced reduction in snow cover or dry soil conditions
must be rejected based on evidence and physical understanding regarding
conditions associated with this particular heatwave event.

First, as noted above, much of the region which experienced record heat
does not normally have snow cover in March, thus this mechanism does
not apply for most of the area that experienced record March heat.
Regarding possible soil moisture feedbacks, it is noteworthy that
antecedent precipitation anomalies since October 1, 2011 (Fig. 10) were
not particularly unusual, except over Minnesota. Even there, basic
climatological knowledge indicates that the cold-season correlation
between precipitation and surface variability based on historical data
is negligible, on average (a very different relation than occurs during
summer). This relationship is well-known in physical meteorology,
namely that surface temperature variability during the cold months is
more driven by horizontal advection and large-scale air-mass transports
than by local land surface interactions involving soil moisture. To the
extent that soil moisture is of material relevance to the surface energy
balance in the northern US in March, only Minnesota exhibited appreciable
antecedent dryness.

Figure 11: North American Snow Cover Anomalies for March [Source: Rutgers University Global Snow Lab]

Figure 12: North American Snow Cover Anomalies for May [Source: Rutgers University Global Snow Lab]

Second, the North American trend in March snow cover has been upward,
not downward. This is revealed in the time series of March snow cover
from 1967-2011 (Fig, 11). The principal decline in snow cover extent
emerges in late spring, when the climatological snow extent pushes well
north into Canada. To illustrate that, attached is the May time series
(Fig. 12) of snow cover for 1967-2011, for which a substantial declining
trend is visually evident. Clearly, March and May snow cover changes
have been materially different from each other over North America,
and indeed of opposite sign in recent decades.

Third, the vertical profile of temperature anomalies during March 2012
is physically inconsistent with the conjecture of a strong surface
feedback. As already indicated, 850mb temperature anomalies were +15°C,
associated with strong anomalous southerly flow that tapped southern
U.S. airmasses, and transported warm air rapidly northward.
Sea surface temperatures over the Gulf of Mexico were
about +1°C above normal during this period, and thus would have
explained only a modest portion of the magnitude of the heatwave over the
Upper Midwest. The further analysis of atmospheric soundings and the
vertical profile of temperatures clearly reveals the strong thermal
advection occurring throughout the lower troposphere. The vertical
signature of the temperature anomalies is thus inconsistent
with surface-driven feedbacks. The studies by Kumar et al (2010)
and Screen et al. (2012a, b) reveal that
the vertical pattern of the temperature response to snow cover and sea
ice loss in high latitudes exhibits the strongest warming at the surface
but has virtually no signal above about 1000m above the surface.

To be sure, snow cover was intimately coupled to the unfolding heatwave
of March 2012. An
animation of daily snow cover during mid-March reveals a
rapid retreat in snow cover evidently as a response to a northward surge
of very warm air in strong low level southerly winds, rather than as
an appreciable forcing factor of the warming itself. Feedback from the
rapidly vanishing snow cover very likely contributed to some extent to
the very strong surface warming magnitude over the far northern Plains
and Upper Michigan peninsula. However, given the similar magnitudes of
daily temperature departures occurring over Chicago, where snow cover
is typically not present in mid-to late March (and indeed was absent
during the 2012 heatwave), a substantial effect of the snow cover loss
further north is unlikely.

So, while a quantification of the snow impact on the current heatwave
magnitude cannot be precisely discerned without further diagnosis, it
is very likely that changes in snow were a response to the heatwave,
not a cause for its extreme magnitude.